Methods and Apparatus for Resistive Voltage Sensing in an Isolated Power Distribution Unit

ABSTRACT

Methods and apparatus provide for a primary side circuit including one or more voltage nodes; and a monitoring circuit operating to monitor one or more parameters of the primary side circuit, and including at least one sensing circuit and at least one processing circuit within a secondary side circuit, where the sensing circuit includes a resistor network having an input for receiving a first sensed voltage from a first of the voltage nodes of the primary side circuit, traversing an isolation boundary between the primary side circuit and the secondary side circuit while adhering to a safety specification, which includes a primary-secondary isolation requirement, and having an output for providing a first modified sensed voltage to the processing circuit.

BACKGROUND

The invention relates to voltage sensors, and more particularly relatesto voltage sensors that measure AC mains voltage and output to a circuitthat is safe for an operator to touch (SELV circuit). In one or morespecific applications, the invention relates to highly accurate,inexpensive, and physically small voltage sensors for use in devicesthat are UL 60950-1 compliant, and to UL 60950-1 compliant devices thatuse such sensors. The following description focuses upon use of theinvention in a specific context, namely a UL 60950-1 compliant powerdistribution unit (“PDU”) but the invention is not limited to PDUapplications and can be used in other applications where UL 60950-1compliance or other safety standards are necessary or commerciallyadvantageous.

The UL 60950-1 standard establishes requirements that reduce risks topersons who operate and service information technology equipment (“ITequipment”). Examples of IT equipment are data and text processingmachines, data network equipment, such as routers, telecom switches,servers, modems, and PDUs (discussed in more detail below), but ITequipment is intended to be interpreted in the broadest sense and is notlimited to these specifically enumerated devices or to PDUs inparticular.

IT equipment typically derives power from the AC mains supply(“primary”) and contains input/output interfaces (“I/O”) thatinterconnect with other IT equipment. UL 60950-1 requires useraccessible I/O to be safe to touch. A safe to touch circuit is definedby UL 60950-1 as a “secondary extra low voltage” circuit (“SELV”).According to UL 60950-1, a SELV circuit must satisfy these requirements:(a) has no direct connection to a primary and derives its power from atransformer, converter or equivalent isolation device, (b) is limited to42.4 V peak, and (c) insures that requirements (a) and (b) are met undernormal operating conditions and single fault conditions.

IT equipment rooms (also known as data centers) utilize hundreds or eventhousands of units of IT equipment. Each piece of IT equipment receivesprimary power by plugging into an outlet of a power distribution unit(“PDU”). A PDU is also a piece of IT equipment and it typicallyincludes: (a) a high power inlet from which it receives power (typicallyfrom a panel board), (b) multiple lower power outlets, and (optionally)(c) circuit breakers or fuses to protect the outlets from over currentconditions (short circuits, etc.).

PDUs designed for IT equipment rooms advantageously perform functionsadditional to power distribution. For example, intelligent PDUs canreport certain status information over a communication and/orinput/output interface, including: (a) the voltage being supplied to thePDU's inlet, (b) how much power (power=voltage times current) is flowingin the inlet and each outlet, and (c) the trip state (whether voltage ispresent) of each circuit breaker. Since gathering the above statusinformation relies on sensing voltage, an IT equipment room withthousands of units of IT equipment will therefore require thousands ofsuch voltage sensors. It will therefore be evident that requirements forsuch voltage sensors should include: (a) the ability to measure aprimary voltage and output to a SELV circuit, (b) highly accurateoutput, (c) low cost, and (d) small size.

Conventionally, voltage sensors able to measure voltage in a primarycircuit and output the measurement to a SELV circuit have been builtusing transformers, opto-coupler devices, Hall effect devices, etc.These devices are used in order to meet the primary to secondaryisolation requirements of a SELV circuit (which are, again, establishedby the particular standard at issue, such as UL 60950-1). However, thesedevices do not make highly accurate sensors, and are expensive and largein size.

Reference is made to FIGS. 1 and 2, which schematically illustrate asafety compliant power distribution unit (PDU), including conventionalvoltage sensors to achieve primary to secondary isolation. The systemincludes a power distribution unit (PDU) (2), which receives primary ACmains power from an inlet (1). A measurement of the voltage and power atthe inlet (1) is made using one or more voltage sensors (5). Primaryvoltage from the inlet (1) is wired to the inputs of one or more circuitbreakers (3) (if any), or other over-current protectors, such as fuses(not shown). The purpose of each circuit breaker (3) is to limit theelectrical current flowing in the associated outlet receptacles (4) byswitching off voltage (interrupting the current path) when the currentflowing through the given circuit breaker (3) exceeds its rating. Theon/off (“trip”) state of a given one or more of the circuit breakers (3)can be detected using one or more voltage sensors (6) to sense presenceof primary voltage at the output of the circuit breaker (6). Primaryvoltage from the output of each circuit breaker (6) is wired to one ormore outlet receptacles (4). A unit of IT equipment (8) can receivepower from the PDU (2) by connecting an inlet plug (9) of the ITequipment (8) into one of the outlet receptacles (4) of the PDU (2). Ameasurement of the voltage at the outlet receptacle (4), and power drawnby the IT equipment (8), is made using one or more voltage sensors.

Conventional voltage sensors that may be used to perform the voltage andpower measurements in FIG. 1 are shown in FIG. 2. The voltage and powerof the inlet (1) is shown being measured using a SELV circuit (8) thatuses a step down transformer voltage sensor (6) and a current sensor (7)to compute power using the well known electric power formula(power=voltage times current). The step-down transformer (6) meets theisolation requirements of a SELV circuit by using a magnetic field toisolate its input connection to the primary side, lines (2 a and 3) fromits output (6 a) connection to the SELV circuit (8). The voltagerequirements of a SELV circuit are met by using a winding ratio thatreduces its output (6 a) voltage to less than 42.4V peak. In addition, afuse (9 a) is usually included to prevent a short circuit in the eventof a fault in the step down transformer voltage sensor (6).

Disadvantages of the step down transformer (6) include its large size,high cost and significant inaccuracies. The step down transformer (6) islarge and expensive because of a number of turns of magnet wire requiredto handle the high voltage and low frequency of the primary AC voltage.The step down transformer (6) is inaccurate because its magneticinductive coupling results in output amplitude and phase shift variancebetween different transformers of the same make and model number.

The on/off state of each circuit breaker (3) is monitored with aseparate SELV circuit (12). The SELV circuit (12) uses an opticalisolator (10) as a voltage sensor and this meets the isolationrequirements of a SELV circuit by using light to isolate its input (10a) connection to the primary side lines (2 b and 3) from its output (10b) connection to the secondary side SELV circuit (12). The lightemitting diode (“LED”) (10 a) of the optical isolator (10) is wired inseries with a current limit resistor (11) and these two devices are thenwired across the primary output (2 b) of the circuit breaker (10) andthe primary line (3). When the circuit breaker (10) is closed and in thenormal operating state, the LED (10 a) turns on and off once everyprimary AC voltage cycle. When the LED (10 a) is on, it emits photonswhich turn on the transistor (10 b) of the optical isolator (10). Whenthe circuit breaker (10) is open (“tripped”), no LED (10 a) currentflows and the transistor (10 b) remains turned off. The SELV circuit(12) detects whether or not the transistor (10 b) is turning on and offas an indication of the trip state of the circuit breaker (10).

Among the disadvantages of the optical isolator (10) is the relativelylarge power required to turn on its LED (10 a). For example, an LEDrequiring 1 mA of current would require 0.250 watts when used to measurea 250V primary AC mains line. Optical isolators are also inherentlyinaccurate, especially over temperature, and are relatively unreliableas compared with, for example, a simple resistor network.

The voltage and power of each outlet receptacle (5) in FIG. 2 is shownusing a primary powered measurement circuit (13) that uses a resistorvoltage sensor (12) and current sensor (7). However, because theresistor voltage sensor (12) is not isolated from the primary side ACpower (2 b), the primary circuit (13) requires isolation circuitry (14),such as an optical isolator or other type of circuit, to connect it tothe SELV circuitry (15).

The disadvantages of the primary powered resistive sensor measurementcircuit (13) combined with the isolation circuitry (14) are, again, itscost, complexity, accuracy and/or reliability issues.

Although resistor sensors are known to exhibit inherent linearity, highaccuracy, low cost and small size, such sensors have not been used toprovide sensed voltages across isolation boundaries in circuitsrequiring isolation from primary to secondary (such as SELV circuits).Indeed, the accepted wisdom in the circuit design arts is exactlyopposite; namely, to avoid resistive sensing networks in suchapplications. Such accepted wisdom has been developed over years andyears of ingrained group-thinking (which has been passed from master toapprentice) that the use of resistive networks would fail to meetsafety/isolation standards, such as those required by UL 60950-1.Consequently, there are no known circuits in the prior art employingresistive networks to provide sensed voltages across isolationboundaries. Moreover, owing to the accepted wisdom in this art area,skilled artisans are not motivated to use resistive networks in suchapplications. Thus, a long felt, but unsatisfied, need has developed inthis area of circuit design, which has been simply accepted by thoseskilled in the art.

SUMMARY OF THE INVENTION

Again, in one or more specific embodiments, the invention may provide ahighly accurate, inexpensive, and physically small voltage sensor toprovide sensed voltages across an isolation boundary in a circuitrequiring isolation from primary to secondary (such as set forth in UL60950-1). It bears repeating, however, that it is contemplated that theinvention may be embodied in any number of circuits, systems, devices,etc. where UL 60950-1 compliance or other safety standards are necessaryor desired.

One or more aspects of the invention proceed from the entirelyunexpected discovery that a voltage sensor, if properly designed using aplurality of resistors configured as a voltage divider, can satisfyknown safety/isolation requirements (such as the UL 60950-1 SELVrequirements). In this regard, it has been discovered that a voltagesensor designed using a plurality of resistors in a particular way cansatisfy at least the following additional safety/isolation requirements:

-   1. The voltage sensor resistors may connect between one or more    nodes on the primary side and one or more nodes of the secondary    side (e.g., the SELV) provided that minimum clearance and creepage    spacing requirements are met (for example the requirements set forth    in UL 60950-1 section 1.5.7). This requirement may be satisfied, for    example, by constructing the voltage sensor resistors on a printed    circuit board where the distance between components meets the    desired clearance and spacing values.-   2. Under normal operating conditions, or when any single component    (in this case resistors) in the sensor fails due to an open or short    circuit, the current flow from the primary side circuit to the    secondary side circuit (e.g., SELV circuit) must be less than a    specified threshold level (e.g., 700 microamperes peak as specified    in UL 60950-1 sections 1.5.7 and 2.4). This requirement may be    satisfied by using a plurality of resistors wired in series, each of    sufficiently high resistance such that if any one of the resistors    is shorted or opened, the resulting current flow from the primary    side circuit to the secondary side circuit is less than the    threshold level, e.g., 700 microamperes peak.-   3. The resistors must not break down or short when subjected to a    high voltage applied to the highest potential source in the primary    side circuit (the so-called hipot). By way of example, a hipot is    specified in UL 60950-1 section 5.2. This requirement may be    satisfied by choosing resistors with sufficiently high working    voltages.-   4. The voltage one any node of the secondary side circuit (e.g., the    SELV circuit) may not exceed a particular threshold level (e.g.,    42.4V peak as specified in UL 60950-1 section 2.2). This requirement    may be met by choosing the ratio-metric values of the resistors in    the voltage divider to limit the SELV voltage to the threshold    level, e.g., 42.4V peak.

In accordance with one or more aspects of the present invention, anapparatus includes: a primary side circuit including one or more voltagenodes; and a monitoring circuit operating to monitor one or moreparameters of the primary side circuit, and including at least onesensing circuit and at least one processing circuit within a secondaryside circuit. The sensing circuit may include a resistor network havingan input for receiving a first sensed voltage from a first of thevoltage nodes of the primary side circuit, traversing an isolationboundary between the primary side circuit and the secondary side circuitwhile adhering to a safety specification, which includes aprimary-secondary isolation requirement, and having an output forproviding a first modified sensed voltage to the processing circuit.

The resistor network preferably includes: a plurality of series-coupledresistors, which are connected at one end to the first voltage node ofthe primary side circuit, and are connected at an opposite end to ajunction node; and a shunt resistance coupled from the junction node toa reference potential. The output providing the first modified sensedvoltage to the processing circuit is at least one of taken from, andderived from, a voltage at the junction node.

In accordance with one or more embodiments, the first voltage node ofthe primary side circuit exhibits a single ended voltage potential withrespect to the reference potential; and the output providing the firstmodified sensed voltage to the processing circuit is a single endedvoltage taken from the junction node with respect to the referencepotential.

In accordance with one or more further embodiments, the first voltagenode of the primary side circuit exhibits a single ended alternatingcurrent (AC) voltage potential with respect to the reference potential;the apparatus further includes a switching circuit including an inputterminal coupled to the junction node and an output terminal, whichpulses in response to the AC potential at the input terminal; and theoutput providing the first modified sensed voltage to the processingcircuit is a single ended pulsed voltage taken from the output terminalof the switching circuit with respect to the reference potential. Thesingle ended pulsed voltage indicates the presence or absence of the ACvoltage potential of the primary side circuit.

The switching circuit may include a switching transistor having an inputterminal and two output terminals; the input terminal of the switchingtransistor is the input terminal of the switching circuit; one of theoutput terminals of the switching transistor is held at a bias voltagepotential; the single ended pulsed voltage is taken from the other ofthe output terminals of the switching transistor with respect to thereference potential. For example, the switching transistor may be abipolar junction transistor, having a base as an input terminal, anemitter coupled to the reference potential and a collector from whichthe single ended pulsed voltage is taken.

In accordance with one or more further embodiments of the presentinvention: first and second voltage nodes of the primary side circuitproduce a differential voltage; the plurality of series-coupledresistors includes first and second pluralities of series-coupledresistors; the first plurality of series-coupled resistors are connectedat one end to the first voltage node of the primary side circuit, andare connected at an opposite end to a first junction node; and thesecond plurality of series-coupled resistors are connected at one end tothe second voltage node of the primary side circuit, and are connectedat an opposite end to a second junction node.

In accordance with some aspects, the shunt resistance may include firstand second resistances, the first resistance coupled from the firstjunction node to the reference potential, and the second resistancecoupled from the second junction node to the reference potential; andthe output providing the first modified sensed voltage to the processingcircuit may be taken as a differential output between the first andsecond junction nodes.

In accordance with alternative or additional aspects, the apparatusfurther comprises a differential to single ended conversion circuithaving first and second input terminals and an output terminal; theshunt resistance includes first and second resistances; the firstresistance is coupled from the first junction node to a first potential,and the first junction node is coupled to the first input terminal ofthe differential to single ended conversion circuit; the secondresistance is coupled from the second junction node to a secondpotential, and the second junction node is coupled to the second inputterminal of the differential to single ended conversion circuit; and theoutput providing the first modified sensed voltage to the processingcircuit is taken as a single ended output at the output terminal of thedifferential to single ended conversion circuit with respect to thereference potential.

The differential to single ended conversion circuit may include anoperational amplifier having first and second input terminals and anoutput terminal, which are the first, second and output terminals of thedifferential to single ended conversion circuit, respectively; the firstpotential is at a voltage potential above the reference potential; thesecond resistance is coupled from the second junction node to the outputterminal of the operational amplifier; and the single ended output istaken at the output of the operational amplifier with respect to thereference potential.

In accordance with one or more further aspects of the present invention,the one or more voltage nodes of the primary side circuit are coupled toa source of power and the monitoring circuit operates to monitor one ormore parameters of the source of power.

In accordance with one or more further aspects of the present invention,the sensing circuit does not employ any optical devices, transformerdevices, and/or Hall effect devices in traversing the boundary from thefirst sensed voltage to the first modified sensed voltage.

Other aspects, features, and advantages of the present invention will beapparent to one skilled in the art from the description herein taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary and non-limiting embodiments of the invention are illustratedin the figures. The drawings may not be to scale, various details may beenlarged or reduced for clarity, and the illustrate values of anyelectrical components are merely exemplary and not limiting.

FIG. 1 schematically illustrates a safety compliant power distributionunit (PDU) consisting of an inlet, outlets, circuit breakers and voltagesensors for measuring power and detecting circuit breaker open/closestate.

FIG. 2 schematically illustrates conventional voltage sensors that usetransformers and opto-isolators in order to achieve primary to secondaryisolation.

FIG. 3 schematically illustrates a preferred embodiment of the inventionfor a voltage sensor with a single ended primary side sensed input and asingle ended output for connection to a secondary side circuit.

FIG. 4 schematically illustrates a preferred embodiment of the inventionfor a voltage sensor with a differential primary side sensed input and adifferential output for connection to a secondary side circuit.

FIG. 5 schematically illustrates a preferred embodiment of the inventionfor a voltage sensor with a differential primary side sensed input andsingle ended output for connection to a secondary side circuit.

FIG. 6 schematically illustrates a preferred embodiment of the inventionfor a voltage sensor to sense the presence or absence of AC voltage viaa single ended sensed input on a primary side and a pulse wave outputfor connection to a secondary side circuit.

FIG. 7 is a flow chart showing the operation of the embodiment of FIG.6.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Although one or more embodiments of the invention may be designed foruse in a PDU intended for IT equipment applications, and is hereillustrated as used in such a PDU, this is not required. Various aspectsof the invention are suitable for use in any application requiring aninexpensive, accurate, small and low power consumption voltage (orcurrent) sensor that measures the voltage or current of a primary sidesource and outputs the measured value(s) across an isolation boundary toa secondary side circuit, such as a UL 60950-1 compliant SELV circuit.

FIG. 3 illustrates a preferred embodiment of the invention in a system100, that includes a voltage sensor that performs a single endedmeasurement between a primary side AC main line (1) and ground (107) andoutputs a scaled down AC voltage single ended SELV output (108). Asingle ended input, single ended output sensor is useful when measuringa primary 4-wire 3-phase AC power line (1), which includes lines (1 a, 1b, 1 c, 1 d) using a SELV circuit (109) that incorporates single endedanalog to digital converters. By way of example, the Analog DevicesADE7878 Energy Metering IC may be used as the SELV 109. Three identicalvoltage sensors (100 a, 100 b, 100 c) may be used, and thus only adetailed description of voltage sensor 100 a is described in thefollowing paragraphs.

Voltage sensor (100 a) is used in SELV circuits that measure voltage andpower and eliminates the need for the conventional voltage sensors, suchas step down transformers (6) and other isolated primary circuits shownin FIG. 2. Voltage sensor 100 a has the advantages of being smaller,less costly and more accurate than conventional voltage sensors used toprovide measured voltage to SELV circuits. The components of the voltagesensor 100 a may include small surface mount resistors and (optionally)one small surface mount capacitor. Thus, the total cost for the voltagesensor (100 a) may be less than about $0.10 in parts. The sensor isextremely accurate and exhibits precise amplitude and phase response.

Voltage sensor 100 a is a voltage divider including a series resistancenetwork (103), shunt resistance (104) (only one resistor required inthis embodiment), and an optional shunt capacitance, which may beimplemented using a single capacitor (105). The shunt capacitor (105) isonly required when the SELV circuit (109) requires its input to befrequency limited by a low pass filter.

Series resistance (103) is made up of a plurality of resistors. In thisembodiment, which is intended to meet the requirements of UL 60950-1(and the specific line-ground voltage characteristics of the source),seven identical 1.5 megohm, 800 working volt resistors are employed toimplement the series resistance (103). However, it is understood thatthe exact number of resistors, their resistance values, and voltageratings may vary providing they satisfy the requirements of the givensafety standard, in this case UL 60950-1. Series resistance (103)connects to the primary side on one end (1 a), and connects to the SELV(109) of the secondary side on the other end (108).

A resistance connecting the primary side to the SELV (109) is permittedin UL 60950-1 providing it meets certain requirements. Series resistance(103) satisfies UL 60950-1 as follows: (1) the component resistors ofthe series resistance (103) are mounted on a printed circuit board (notshown) where the distance between components meets UL 60950-1 clearanceand spacing values; (2) when any one of the component resistors of theseries resistance (103) fails due to an open or short circuit, thecurrent flow from the primary side (1 a) to the output (108), which isinput to the SELV circuit (109) is less than 700 microamperes peak; and(3) the breakdown voltage of series resistance (103) is 5600V—the sum ofthe working voltages of the seven component resistors wired in series.This breakdown voltage satisfies the electric strength test (hipot)requirement of UL 60950-1 section 5.2.

Shunt resistor (104) reduces the primary voltage so that it does notexceed the 42.4V peak maximum specified in UL 60950-1 section 2.2. Thevoltage reduction uses the well known voltage divider formula:ratio=shunt/(shunt+series). A preferred value of 7.87 k for resistor(104) results in a ratio of 0.000749, which reduces a 250V AC primaryvoltage (1 a) down to a 0.187 volt AC signal on output (108), which issuitable for the Analog Devices ADE 7878 Energy Metering IC, which asdiscussed above may be used to implement the SELV circuit (109).

The shunt capacitor (105) implements an inexpensive first order low passanti-alias filter for the SELV circuit (109), which requires its voltageinputs to be frequency band limited. The −3 dB cut off frequency of thelow pass filter occurs when the magnitude of the capacitor's impendenceequals the resistance of the shunt resistor (104) using the well knowncapacitor impedance formula: Z=1/(2π*frequency*capacitance). Thus, forthe 7.87 k shunt resistance (104) and a 4 nanofarad capacitance (105),the −3 dB cut off frequency is approximately 5 kHz, which is suitablefor use with the Analog Devices ADE 7878 Energy Metering IC (109).

FIG. 4 illustrates a preferred embodiment of the invention asimplemented in a system (200) including a voltage sensor that performs adifferential measurement across two primary side AC mains lines (5 a, 5b) and outputs a scaled down differential AC voltage output (6 a, 6 b)for connection to a secondary side SELV circuit (204). A differentialoutput sensor is useful with SELV circuits (204) that incorporatedifferential input analog to digital converters. For example, the SELVcircuit (204) may be implemented using the Analog Devices ADE 7763energy meter integrated circuit. This type of sensor is used in SELVcircuits that measure voltage and power and avoids the need forconventional voltage sensors, such as the step down transformers andisolated primary circuits shown in FIG. 2.

This voltage sensor has the advantages of being smaller, less costly andmore accurate than conventional voltage sensors used in suchapplications where isolation is required or desired. The components ofthe system 200 preferably include a plurality of small, surface mountresistors (201 a, 201 b, 202 a, and 202 b) and (optionally) a pluralityof small, surface mount capacitors (203 a, 203 b). The total part costfor components shown is less than about $0.20. The voltage sensor isextremely accurate and exhibits precise amplitude and phase response.

The voltage sensor includes two identical voltage dividers, where eachdivider contains a series resistance (201 a and 201 b), a shuntresistance (202 a and 202 b), and a shunt capacitance (203 a and 203 b).The shunt capacitance, which in this case is implemented as a singlecapacitor (203 a and 203 b) in each voltage divider, are only requiredwhen the SELV circuit (204) requires its input to be frequency limitedby a low pass filter.

Series resistors (201 a and 201 b) are each made up of a plurality ofseries-coupled resistors, for example seven identical 1.5 megohm, 800working volt resistors. Again, although the exact number of resistors,their values and voltage ratings may vary, the combination shouldsatisfy the requirements of the particular safety standard at issue, inthis example, UL 60950-1.

Each series resistance (201 a and 201 b) satisfies UL 60950-1 asfollows: (1) the component resistors of each series resistance aremounted on a printed circuit board (not shown) where the distancebetween components meets UL 60950-1 clearance and spacing values; (2)when any one of the component resistors of either series resistance (201a and 201 b) fails due to an open or short circuit, the current flowfrom the primary side (205 a and 205 b) to the outputs (206 a, 206 b),which are input to the SELV circuit (204) is less than 700 microamperespeak; and (3) the breakdown voltage of each series resistance (201 a,201 b) is 5600V—the sum of the working voltages of the series-coupledcomponent resistors in each resistance (201 a, 201 b). This breakdownvoltage satisfies the electric strength test (hipot) requirement of UL60950-1 section 5.2.

Shunt resistors (202 a and 202 b) reduce the primary voltage so that itdoes not exceed the 42.4V peak maximum specified in UL 60950-1 section2.2. The voltage reduction uses the well known voltage divider formula:ratio=shunt/(shunt+series). The preferred values of 7.87 k for resistors(202 a, 202 b) results in a ratio of 0.000749 which will reduce a 250VAC primary voltage (205 a and 205 b) down to a 0.187 volt AC signal oneither of lines (206 a and 206 b), which is suitable for the AnalogDevices ADE 7763 Energy Metering IC SELV circuit (204).

The shunt capacitors (203 a and 203 b) implement an inexpensive firstorder low pass anti-alias filter for SELV circuits that require theirinputs to be frequency band limited. The −3 dB cut off frequency of thelow pass filter occurs when the impendence magnitude of the capacitorequals the resistance of the shunt resistor (202 a or 202 b) using thewell known impedance formula for capacitors:f=1/(2π*frequency*capacitance). For the 7.87 k shunt resistance (202 aand 202 b) and a 4 nanoFarad capacitance (203 a and 203 b), the −3 dBcut off frequency is approximately 5 kHz which is suitable for use withthe Analog Devices ADE 7763 Energy Metering IC (204).

FIG. 5 illustrates a preferred embodiment of the invention asimplemented in a system 300 including a voltage sensor that performs adifferential measurement across two primary side AC mains lines (305 a,305 b) and outputs a scaled down AC voltage single ended output on line(306) for input to a SELVE circuit (304). A single ended output sensoris useful with SELV circuits (304) that incorporate single ended inputanalog to digital converters. By way of example, the SELV circuit 304may be implemented using a general purpose microprocessor, like the STMicroelectronics STM32 microcontroller integrated circuit.

This type of sensor is used in SELV circuits that measure voltage andpower, and avoids the need for conventional voltage sensors, such asstep down transformers and isolated primary circuits shown in FIG. 2.This sensor has the advantages of being smaller, less costly and moreaccurate than conventional voltage sensors used to provide measuredvoltages across isolation boundaries. The components of the voltagesensor includes a plurality small, surface mount resistors (301 a, 301b, 302 a, 302 b), a general purpose operational amplifier (307), and(optionally) a plurality of small, surface mount capacitors (303 a, 303b). The total parts cost for these components is less than about $0.45.The sensor is extremely accurate and exhibits precise amplitude andphase response.

The voltage sensor preferably includes two identical voltage dividerswhere each divider contains a series resistance (301 a and 301 b), ashunt resistance (302 a and 302 b), and an (optional) shunt capacitance(303 a and 303 b).

Series resistances (301 a and 301 b) are each made up of a plurality ofseries-coupled resistors. By way of example, each series resistance (301a and 301 b) may include seven identical 1.5 megohm, 800 working voltresistors. Again, although the exact number of resistors, their valuesand voltage ratings may vary, they are intended to satisfy therequirements of the particular safety standard at issue, in this case UL60950-1. Series resistances (301 a and 301 b) each satisfy UL 60950-1 asfollows: (1) the component resistors of each series resistance aremounted on a printed circuit board (not shown) where the distancebetween components meets UL 60950-1 clearance and spacing values; (2)when any one of the component resistors of either series resistance (301a and 301 b) fails due to an open or short circuit, the current flowfrom the primary side (305 a and 305 b) to the output (306), which isinput to the SELV circuit (304) is less than 700 microamperes peak; and(3) the breakdown voltage of each series resistance (301 a, 301 b) is5600V—the sum of the working voltages of the series-coupled componentresistors in each resistance (301 a, 301 b). This breakdown voltagesatisfies the electric strength test (hipot) requirement of UL 60950-1section 5.2.

Shunt resistances (302 a and 302 b), which are implemented in thisexample by respective, single resistors, reduce the primary voltage sothat it does not exceed the 42.4V peak maximum specified in UL 60950-1section 2.2. The shunt capacitance (303 a and 303 b), which areimplemented in this example by respective, single capacitors, result inan inexpensive first order low pass anti-alias filter for SELV circuitsthat require their inputs to be frequency band limited. The −3 dB cutoff frequency of the low pass filter occurs when the impendence of agiven capacitor (303 a, 303 b) equals that of the respective shuntresistor (302 a or 302 b) using the well known impedance formula forcapacitors: Z=1/(2π*frequency*capacitance). For a 47 k shunt resistancefor each resistor (302 a and 302 b), and 680 picofarad capacitance foreach capacitor (303 a and 303 b), the −3 dB cut off frequency isapproximately 5 kHz, which is suitable for use with the STM32 MCU analogto digital converter (304).

The operational amplifier (307) incorporates the two voltage dividersinto a differential amplifier topology. Since the values of the seriesresistances (301 a, 301 b) are identical and the values of the shuntresistances (302 a, 302 b) are identical, the output of the operationalamplifier (307) adheres to the well known differential operationalamplifier gain formula: output=input*shunt/(shunt+series). For preferredvalues of 1.5 megohm for each resistor of resistances (301 a, 301 b),and 47 k for each resistance (302 a, 302 b), the ratio equals 0.0045,which will reduce a 250V AC primary voltage differential across lines(305 a, 305 b) down to a 1.1 volt AC signal on line (306), which issuitable for an STM32 microcontroller single ended SELV circuit (304).

FIG. 6 illustrates a preferred embodiment of the invention implementedin a system 400 including a voltage sensor that senses the presence orabsence of AC voltage by performing a single ended measurement between aprimary side AC main line (407) and ground (408) and produces a pulsewave output (404) for input to a secondary side circuit, such as a SELVcircuit 405, when the AC voltage is above a prescribed amplitudethreshold. This type of voltage sensor is used in SELV circuits thatdetect the presence or absence of primary AC voltage, such as blownfuses, tripped circuit breakers or any other type of on/off switched ACvoltage.

This voltage sensor of the system 400 has the advantages of beingsmaller, less costly and using less power than conventional voltagesensors, such as the optical isolator (10) shown in FIG. 2. Thecomponents of the system 400 include a plurality of small, surface mountresistors (401, 402), and one or more transistors (403), in thisexample, one transistor. The total cost for the voltage sensor is lessthan about $0.10. The sensor draws approximately 5 milliWatts of powerfrom the primary AC power line, which is much less than the 230milliWatts typically required for the optical isolator (10).

The voltage sensor includes a voltage divider, comprising a seriesresistance (401) and shunt resistance (402). The series resistance (401)is preferably made up of a plurality of series-coupled resistors, suchas seven identical 1.5 megohm, 800 working volt resistors. Again,although the exact number of resistors, their values and voltage ratingsmay vary from application to application, the result is intended tosatisfy the requirements of applicable safety standard, such as the UL60950-1. The series resistance (401) connects to the primary side on oneend (407), and to the secondary side on the other end (410), which iscoupled to the SELV circuit (405). A resistive network connecting aprimary side to a secondary side SELV circuit, across an isolationboundary, is permitted in UL 60950-1, providing it meets certainrequirements. The resistance (401) satisfies UL 60950-1 as follows: (1)the component resistors of the series resistance (401) are mounted on aprinted circuit board (not shown) where the distance between componentsmeets UL 60950-1 clearance and spacing values; (2) when any one of thecomponent resistors of the series resistance (401) fails due to an openor short circuit, the current flow from the primary side (407) to theoutput (404), which is input to the SELV circuit (405) is less than 700microamperes peak; and (3) the breakdown voltage of series resistance(401) is 5600V—the sum of the working voltages of the seven componentresistors wired in series.

Shunt resistance (402), which in this case is implemented with a singleresistor, reduces the primary voltage so that it does not exceed the42.4V peak maximum specified in UL 60950-1 section 2.2. The voltagereduction uses the well known voltage divider formula:ratio=shunt/(shunt+series). For preferred values of 1.5 megohm for eachresistor of series resistance (401), and 100 k for resistance (402), theratio equals 0.0095, which will reduce a 250V AC primary voltage (407)down to a 2.38 volt AC signal on node (410).

The base of bipolar transistor (403) is wired to the voltage divideroutput (410), and the bipolar transistor (403) turns on when the voltagedivider output is greater than about 0.6 volts. The ratio of the seriesand shunt resistances (401, 402) is chosen such that any primary ACvoltage (407) greater than about 70 volts will produce an output voltage(410) greater than the 0.6 volts required to turn on the transistor(403). When the circuit breaker (409) is closed and in the normaloperating state, the transistor (403) turns on and off once everyprimary AC voltage cycle. When the circuit breaker (409) is open(“tripped”), no primary AC voltage is present at its output (407) andthe bipolar transistor (403) remains turned off.

The output of the transistor (403) is connected to a general purposeinput/output (GPIO) pin (406) of a microprocessor, such as an STMicrosystems STM32, which is suitable to implement the SELV (405). Thegeneral purpose microprocessor is programmed with an algorithm to detectthe presence or absence of a pulse wave (404) on the GPIO input pin(406). Presence of the pulse wave (404) is interpreted as circuitbreaker closed. Absence of the pulse wave (404) is interpreted ascircuit breaker open (“tripped”).

A preferred microprocessor algorithm to determine whether the circuitbreaker (409) in FIG. 6 is open or closed is shown as a flowchart inFIG. 7. The programming steps in the algorithm are carried out over ameasurement period, e.g., about one second long, and immediatelyrepeated once a measurement has been concluded. In first step 101, anaccumulator register and a counter register are set to zero. Thealgorithm then pauses for 100 microSeconds in step 110. The algorithmthen checks the logic state of the GPIO pin in step 120. If the GPIO pinis a logical “0”, which indicates the presence of a pulse wave, theaccumulator is incremented in step 130. If the GPIO pin is a logical “1”which indicates the absence of a pulse wave, the accumulator is notincremented. The algorithm then increments the counter (step 140) andthe value of the counter is checked in step 150. If the counter is lessthan 10,000, the algorithm repeats from step 110. If the counter isequal to 10,000 it indicates that 10,000 checks of the GPIO pin havebeen performed over a one second period and the algorithm proceeds tocheck the value of the accumulator in step 160. If the value of theaccumulator is greater than 1,000, it indicates the presence of a pulsewave on the GPIO pin with a duty cycle of at least 10 percent and thealgorithm outputs a “circuit breaker closed” indication in step 170. Ifthe value of the accumulator is less than 1,000, it indicates absence ofa pulse wave of the GPIO pin and the algorithm outputs a “circuitbreaker open” indication in step 180.

The algorithm shown in FIG. 7 is robust and immune to noise present onthe primary AC voltage because it uses a decision algorithm based on10,000 measurements over a one second time period.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

What is claimed is:
 1. An apparatus, comprising: a primary side circuitincluding one or more voltage nodes; a secondary side circuit includingat least one processing circuit; and a resistor network having an inputfor receiving a first sensed voltage from a first of the one or morevoltage nodes of the primary side circuit, traversing an isolationboundary between the primary side circuit and the secondary side circuitwhile adhering to a safety specification which includes aprimary-secondary isolation requirement, and having an output forproviding a first modified sensed voltage to the processing circuit; andwherein said resistor network does not bridge any isolation circuit. 2.The apparatus of claim 1, wherein the resistor network includes: aplurality of series-coupled resistors, which are connected at one end tothe first voltage node of the primary side circuit, and are connected atan opposite end to a junction node; a shunt resistance coupled from thejunction node to a reference potential; and the output providing thefirst modified sensed voltage to the processing circuit is at least oneof taken from, and derived from, a voltage at the junction node.
 3. Theapparatus of claim 2, wherein: the first voltage node of the primaryside circuit exhibits a single ended voltage potential with respect tothe reference potential; and the output providing the first modifiedsensed voltage to the processing circuit is a single ended voltage takenfrom the junction node with respect to the reference potential.
 4. Theapparatus of claim 2, wherein: the first voltage node of the primaryside circuit exhibits a single ended alternating current (AC) voltagepotential with respect to the reference potential; the apparatus furtherincludes a switching circuit including an input terminal coupled to thejunction node and an output terminal, which pulses in response to the ACpotential at the input terminal; and the output providing the firstmodified sensed voltage to the processing circuit is a single endedpulsed voltage taken from the output terminal of the switching circuitwith respect to the reference potential.
 5. The apparatus of claim 2,wherein: first and second voltage nodes of the primary side circuitproduce a differential voltage; the plurality of series-coupledresistors includes first and second pluralities of series-coupledresistors; the first plurality of series-coupled resistors are connectedat one end to the first voltage node of the primary side circuit, andare connected at an opposite end to a first junction node; and thesecond plurality of series-coupled resistors are connected at one end tothe second voltage node of the primary side circuit, and are connectedat an opposite end to a second junction node.
 6. An apparatus,comprising: a primary side circuit including one or more input terminalsfor operative connection to a source of power; a secondary side circuitincluding at least one processing circuit; and a monitoring circuitreceiving a first sensed voltage from one of the input terminals of theprimary side circuit, traversing an isolation boundary between theprimary side circuit and the secondary side circuit while adhering to atleast a safety-extra-low-voltage (SELV) specification, and providing afirst modified sensed voltage to the processing circuit, wherein themonitoring circuit does not bridge any isolation circuit.
 7. Anapparatus, comprising: at least one primary side circuit including oneor more input terminals for operative connection to a source of power;at least one secondary side circuit including at least one processingcircuit; a boundary providing isolation between the at least one primaryside circuit and said at least one secondary side circuit, where theboundary adheres to at least a safety-extra-low-voltage (SELV)specification; and at least one monitoring circuit receiving a firstsensed voltage from one of the input terminals of the primary sidecircuit, traversing the boundary while adhering to the SELVspecification, and providing a first modified sensed voltage to theprocessing circuit of the secondary side circuit, wherein the monitoringcircuit does not bridge any isolation circuit.